With advances in antibiotics and vaccines there has been a reduction in the seriousness of many infectious diseases; however, cancer still remains as a mostly incurable threat. In fact, cancer accounts for about 10 percent of all deaths in the U.S. every year (Oppenheimer, 1985). One obstacle in the treatment of cancer is that the basic mechanism of cancer development and propagation is not well understood and, therefore, investigation into possible cancer treatments may require knowledge from a variety of different disciplines (Braun, 1974, Muir, 1988). Cancer patients must withstand the debilitating mental and physical effects throughout the long duration of the disease which also results in an economic burden to both the patient and the community (Busch, 1974).
The mortality rate for patients diagnosed with either primary or secondary liver cancer is very high. Many new approaches towards possible treatments are currently being investigated; however, successes have been minimal and surgery still remains as the best form of treatment, even though less than 10 percent of the patients are suitable for this option (Kemeny et al., 1995). Non-surgical forms of treatment include various routes of chemotherapy in which toxic chemotherapeutic drugs are delivered to the liver tumors, either systemically (throughout the entire body) or regionally (directly into the liver). The chemotherapeutic drugs such as fluorodeoxyuridine (FUDR) and doxorubicin (adriamyacin) work by having a greater toxic effect on actively dividing cells such as cancer cells, rather than most normal tissues. The goal in this form of treatment is to deliver a high dose of the drug to the tumor tissue while keeping the concentration of the drug (and its toxic effects) in normal tissue to a minimum. The toxic side effects of the chemotherapeutic agents may be the limiting factor in determining the drug concentration delivered to the patient. In many cases there is insufficient killing of the tumor cells and regrowth and spreading may occur (Bhattacharya et al., 1994). In addition, with conventional systemic or regional treatment, the excess drug which does not contact tumor tissue degrades the condition of the healthy tissue and, therefore, can become the limiting factor in dose concentration (Kemeny et al., supra). An ideal situation would occur if the toxic effects of the drugs could be completely localized within the liver tumor tissue without affecting the surrounding healthy tissue, enabling a higher drug concentration to completely kill all of the cancer cells.
Chemotherapy is often combined with another form of treatment termed embolization in which the blood supply to the tumor is essentially reduced or stopped either temporarily or permanently in an attempt to arrest the tumor growth or cause regression. Typical embolic agents include steel coils as well as polyvinyl alcohol sponge (IVALON), collagen, gelatin sponge (GELFOAM), albumin, and starch materials that may be in the form of microspheres or particles. Because healthy liver tissue has a dual blood supply through the hepatic artery and the portal vein, and most hepatic tumors are oxygenated almost exclusively from the hepatic artery, the theory behind embolization is that this artery can be obstructed by injections of these materials in an attempt to starve the tumor of its blood supply without injury to the majority of the liver. If this technique is used in combination with regional chemotherapy the drug can be contained within the tumor tissue for longer periods of exposure time (Lin et al., 1988, Kemeny et al., 1995). In many cases, however, a collateral circulation will appear and circumvent the blockage or the embolized artery will reopen allowing blood to once again feed the tumor. If viable tumor cells still remain, this will allow them to regrow tumor tissue (Kemeny et al., 1995).
Natural and synthetic polymers have been used to produce microspheres for a variety of biomedical applications including general and targeted drug delivery devices. The term microsphere generally refers to spherical particles between 2 nm to 50 nm in diameter but smaller sizes (usually below 1 micrometer) may be referred to as nanospheres. Micro particles are similar but usually irregular in shape (Arshady, 1993). As mentioned previously, some polymeric spheres and particles have been used as embolic agents for the treatment of liver cancer. These materials, such as starch, poly (vinyl alcohol), and gelatin, do not release a drug but rather serve to occlude the blood flow after a drug has already been delivered in order to allow increased retention time within the liver (Lin et al., 1988). Work has also been done in the development of polymeric microspheres that deliver anticancer drugs in a controlled fashion. For example, a feasibility study was done for the oral delivery of an anticancer drug, methotrexate, encapsulated in degradable gelatin microspheres. The microspheres were coated with the natural polymers chitosan and alginate which would enable the microspheres to pass through the gastrointestinal tract to reach the intestine where the drug action or absorption is desired. In theory, higher concentrations of the toxic drug could be delivered using this targeted delivery system rather than systemic treatment while reducing side effects which include vomiting, diarrhea, gastro intestinal ulceration, and liver and kidney damage (Narayani et al., 1995). Experiments by Kato et al. (1981) showed that mitomycin C or cisplatin could be encapsulated within biodegradable ethyl-cellulose microcapsules for possible use in chemoembolization, and a separate study showed that cisplatin could be loaded into poly(lactide) microspheres such that continuous release could be obtained for a period of several days to a week. Cisplatin is one of the most potent chemotherapeutic agents known and is commonly used to treat liver tumors. Since the drug can cause many toxic side effects, the use of microspheres has been suggested to target its action by hepatic arterial injection and controlled release (Spenlehauer et al., 1986). This idea is supported by a separate study using a rat model that showed microspheres of a certain size range delivered to the liver via the hepatic artery were found to be concentrated in a 3:1 ration of tumor tissue to liver tissue for implanted salivary adenocarcinomas (Meade et al., 1987). While these degradable microsphere systems would be able to achieve continuous release within the liver, they remain non-tumor specific and drug concentrations would ultimately be limited by the toxic side effects produced, including damage to healthy liver tissue (Kemeny et al., 1995). One current area of research that attempts to increase the targeting of anticancer treatment is with the use of magnetically directed microspheres. Hafeli et al. (1994) have developed poly(lactic acid) microspheres which can be loaded with Yttrium-90 and incorporated with magnetite such that it may be possible to magnetically direct the radiotoxic effect of the spheres to be more concentrated near tumor sites.
Malignant cells show an increased rate of glucose uptake and aerobic glycolysis with the resulting formation of lactic acid (Volk et al., 1993). In normal cells the uptake of glucose is accomplished by membrane proteins known as glucose transporters. Depending on the cell type, the proteins show different patterns of expression, hormone responsiveness, and transport properties. When cells transform into the malignant state the number of the glucose transporter proteins per cell is commonly increased. Because of this, the uptake of glucose into malignant cells is no longer regulated by systemic or cellular demands, and is instead controlled almost completely by the extracellular concentrations (Jahde and Rajewsky, 1982). This means that the increased amounts of lactic acid produced by the aerobic metabolism can be further increased by the systemic infusion of glucose, resulting in local tumor pH values that are lower than that for healthy tissue (which remains consistently close to 7.4) (Volk et al., 1993).
As can be understood from the above, there remains a need for a drug delivery system for cancer treatment, such as primary or secondary liver cancer, that would release an anticancer agent in high concentrations only within the tumor tissue while healthy tissue would remain relatively unaffected.